There is an association between poor nutritional status and the progression of lung disease in patients with cystic fibrosis (CF), with the most severe decline experienced by post-menarchial females. There is a gender difference in mortality risk that is mediated by nutritional status, and while the association between decreased pulmonary function (PF) and increased resting energy expenditure has been described in males, it has not been described in females. Thus, the factors influencing the decline in nutritional status and PF may be different in males and females. The goal of this study is to describe this association in young women. In addition, other aspects of energy balance (total energy expenditure and dietary intake), nutritional status (body composition and growth), PF, and disease severity are being assessed in order to describe more accurately the relationship between nutrition and disease progression in young women with CF.